The summer transfer window of 2021 has been a rollercoaster ride for all of Europe, especially La Liga and Premier League clubs. Following suit for the past three months, the deadline day was as eventful as they come. Spanish teams were particularly active in the final hour, partaking in some of the strangest and most unexpected transfers in La Liga's history.Some notable outgoing transfers from La LigaThe crazy whirlwind of a window has left some of Europe's biggest powerhouses without their star players. La Liga stalwarts Barcelona and Real Madrid are the perfect examples of this phenomenon. Aside from the massive loss of Lionel Messi and Sergio Ramos, the two Clasico rivals also witnessed the departures of a number of promising youngsters.Future prospects including Ilaix Moriba and Brahim Diaz chose to abandon the riches of La Liga in an attempt to find success elsewhere.Saúl to Chelsea, HERE WE GO! Agreement reached for €5m loan from Atletico Madrid. Personal terms agreed. Saúl will be in London soon to sign the contract. 🔵🤝 #CFC #DeadlineDayFinal details and the deal will be announced. Difficult deal but finally set to be completed. pic.twitter.com/3Jq5e3JwXE— Fabrizio Romano (@FabrizioRomano) August 31, 2021Similar to Madrid and Barca, Spanish giants Sevilla and Getafe also had their camps raided by some major Premier League clubs. Brighton and Hove Albion got hold of Getafe's Marc Cucurella for a nominal fee of €15m while Spurs were able to land the promising Bryan Gil from Sevilla.With so many talents securing a transfer away from La Liga, the Spanish top-flight is in for a tough 2021-22 season, especially on the European front. Nevertheless, this transfer window brought substantial changes to all of the major clubs. While the positive alterations will show their merit in due time, negative ones remain more drastic and apparent.On that note, let's take a look at five La Liga clubs that had the worst outing in the 2021 summer transfer window:#5 Athletic BilbaoAthletic Bilbao had yet another disappointing transfer windowWhilst negotiating a business deal, the concerned parties often try to extract as much profit as they can from their end. However, the concept of profit is absolutely useless if a deal hasn't been struck in the first place. Athletic Bilbao find themselves in a similar scenario, having conducted little to no business in the transfer window gone by.The Basque club officially announced only one player before the commencement of this season. A 24-year-old right-back was the only footballer the La Liga mainstays had signed in the past three months. Alex Petxa from the B team of Real Sociedad joined the depleted Bilbao team on a free transfer in July 2021.PSG reject £189m bid for Kylian Mbappe.To put this into perspective, the amount spent on transfers by Athletic Club de Bilbao, in the ENTIRE HISTORY of the club is around £145m 🤯#DeadlineDay pic.twitter.com/SScfKCuTZ7— Athletic Club UK (@AthleticClubUK) August 31, 2021In addition to this sole incoming transfer, Los Leones also oversaw the departures of three experienced players, including veterans like Iago Herrerin and Ibai Gomez. With a lowly 10th place finish last season, Athletic Bilbao certainly haven't done enough to better their current position.In Marcelino, they do have a competent coach with the capabilities to work wonders. However, with no help from the transfer department, Bilbao fans can expect yet another mediocre season in La Liga.#4 BarcelonaBarcelona lost Lionel Messi to PSG this seasonIf such a list was compiled before August 2021, Barcelona wouldn't be within a mile's radius of being considered as a possible entry. Nevertheless, the past 31 days have been more arduous for the Blaugrana than any other top European side. Above all, the Catalans lost out on their talisman Lionel Messi, having already agreed to personal terms to re-sign their all-time top scorer.Barcelona came into the transfer window with all guns blazing, signing Sergio Aguero, Eric Garcia, Emerson Royal and Memphis Depay for a combined total of €14m. However, that is where the niceties end. The La Liga giants let the promising Francisco Trincao leave on loan to Wolverhampton Wanderers without having a proper plan to structure their attack.Antoine Griezmann back to Atletico Madrid, HERE WE GO! Agreement now completed with Barcelona. Loan until June 2022 with buy option for €40m. ⚪️🔴 #Atleti #DeadlineDayGriezmann has agreement for two years contract plus one option. He’s BACK at Atléti. Shocking move of the day. pic.twitter.com/kBQAkgQyLt— Fabrizio Romano (@FabrizioRomano) August 31, 2021While the sale of three more players did bring in some much-needed revenue, Barcelona failed to get rid of the deadwood that is still burning a hole in their pocket.However, deadline day proved to be the defining factor in deeming their transfer window a failure. Barcelona sold Emerson and Ilaix Moriba in quick succession. They most notably allowed Antoine Griezmann to re-join Atletico Madrid, gravely strengthening their rivals' attack.In response, the feeble loan signing of Luuk de Jong from fellow La Liga side Sevilla was announced. With such bizarre moves, Barcelona successfully ruined what was shaping up to be a decent transfer window for the 26-time La Liga champions.Also read: 5 biggest transfer downgrades of 2021